The present invention relates to a mirror unit of an exposure device used in an image forming apparatus such as a copying machine, a printer, a facsimile machine and a multifunctional peripheral having functions of the aforesaid equipment, and to an image forming apparatus that uses the mirror unit.
Some of image forming apparatuses such as copying machines and printers are equipped with an exposure device that uses a laser beam for scanning. This exposure device is one that causes a light flux emitted from a light source through light modulation to be deflected periodically by a polygon mirror in accordance with image signals, and to be focused, by a scanning lens, to be in a spot shape on a surface of a photoreceptor drum representing a photoconductor, to conduct light scanning on the surface of the photoconductor, for image recording.
Among these exposure devices, there are known some devices each having been downsized by arranging a mirror in an optical path covering from the aforesaid light source to the photoreceptor drum surface, and by creating an optical path structure wherein a light flux emitted from the aforesaid light source is reflected on the mirror to be projected on the surface of the aforesaid photoreceptor drum surface.
A mirror used in the exposure device is equipped with an angle adjusting mechanism by which an optical axis that is reflected by the mirror and points to the aforesaid photoreceptor drum can be adjusted properly, when an angle of a reflection surface of the mirror is adjusted by the aforesaid angle adjusting mechanism.
FIG. 7 shows a conventional mirror angle adjusting mechanism typically in the aforesaid exposure device. Mirror M is arranged by pointing its longitudinal direction to the direction of the scanning optical axis Lo, while fitting its center of gravity G on the scanning optical axis Lo.
The aforesaid mirror M has support point “a” on its one end portion in its longitudinal direction and two support points “b” and “c” on its other end portion in the width direction of the mirror, and thereby, the mirror M is supported at three points on a mirror base member that is in the outside of the drawing.
In the arrangement configuration, the support point “a” on the 1-point supporting side of the aforesaid mirror M is set to the center in the width direction of the mirror M so that it may be arranged on the aforesaid scanning optical axis Lo, while, the support points “b” and “c” on the 2-point supporting side are set on line-symmetric positions about the center represented by the aforesaid scanning optical axis Lo, thereby, the abovementioned center of gravity G is in existence in a triangle area formed by connecting the support points “a”, “b” and “c”.
And, one of the aforesaid support points “b” and “c” on the 2-point supporting side, namely, support point “c”, for example, is configured as a movable adjusting section by an adjust screw, for example.
Owing to the foregoing, when the movable adjusting section (support point “c”) is moved slightly in the direction of a space of FIG. 7, the mirror M is slightly rotated on mirror rotary axis 0 represented by a line connecting the aforesaid support points “a” and “b”, thus a supporting angle of the mirror M is changed, and an optical axis adjustment is carried out.
Related technologies for the angle adjustment for the mirror M are shown, for example, in Unexamined Japanese Patent Application Publication Nos. H11-202409 and 2003-263000.
In the conventional construction shown in FIG. 7, when adjusting an angle of the mirror M, the mirror M makes a twisted rotation because the mirror M cannot rotate on the scanning optical axis Lo, resulting in a possibility that a reflected optical axis is deflected to worsen the accuracy for the optical axis adjustment.
For avoiding this twisted rotation of the mirror M, it is possible to cause a line of mirror rotary axis O that connects these support points “a” and “b” to be adjusted on the scanning optical axis Lo, by setting also support point “b” on one side of the aforesaid the 2-point supporting side, on the scanning optical axis Lo in the same way as in the case of support point “a” on the 1-point supporting side. However, in this way, the center of gravity G exists on the mirror rotary axis O, which makes a vibration to affect easily, to have an adverse effect such as pitch unevenness on images.
Therefore, it is considered that these support points “a” and “b” are shifted in the width direction of the mirror M from the upper portion of the scanning optical axis Lo and the mirror rotary axis O is moved in parallel for the scanning optical axis Lo, so that the center of gravity G of the mirror M may be in the existence of the triangle area formed by connecting three support points “a”, “b” and “c”. In this case, however, support point “a” on the 1-point supporting side is caused naturally by the parallel displacement of the mirror rotary axis O for the aforesaid scanning optical axis Lo to lean in the width direction the mirror M from the center in the width direction of the mirror M, and the center of gravity of the mirror on the 1-point supporting side of the mirror M is caused to lean in the other width direction of the mirror M, which makes the twisted rotation of the mirror M to be caused easily.
With the foregoing as a background, an object of the invention is to provide a mirror unit of an exposure device that can avoid a twisted rotation of a mirror in the case of an angle adjustment of the mirror and to provide an image forming apparatus using the aforesaid mirror unit.